Ink jet printing is performed by discharging ink droplets from a print head to a substrate. The droplets are ejected through orifices or nozzles in the print head and are directed to the substrate to form an image thereon. In contrast to many other types of printing, there is no contact between the printer and the substrate in ink jet techniques.
Most of the ink jet printers known in the art can be characterized as either continuous or impulse devices, depending upon the mechanism by which the ink droplets are directed to the substrate. In continuous ink jet systems, an essentially uninterrupted stream of ink is ejected from a nozzle and breaks up into droplets. The droplets bear an electric charge so that they can be deflected by an applied electric field which is modulated according to the particular image to be recorded. The electric field directs the droplets toward either the substrate or an ink re-circulating reservoir. The inks employed in conjunction with continuous ink jet systems typically comprise a colorant such as a dye or pigment, an electrolyte to facilitate droplet deflection, and a liquid vehicle to dissolve or disperse the colorant and the electrolyte. The vehicle in many continuous-type inks comprises water.
With so-called "impulse" or "drop-on-demand" ink jet printers, image formation is controlled by selectively energizing and de-energizing a piezoelectric transducer rather than by modulating an applied electric field. Ink is stored in the print head or nozzle until it is necessary to form an image on the substrate. The printer is then activated to apply pressure to the ink and discharge a selected number of discrete ink droplets toward the substrate. These ink droplets need not bear an electric charge. Accordingly, impulse ink compositions are free of corrosive substances such as water and electrolytes which continuous stream inks often comprise.
However, impulse ink jet printers present problems that are not encountered in continuous ink jet systems. An important problem is associated with printer stand-by that may cause the ink to solidify at the discharge orifices of the print head. Therefore, an impulse ink jet composition must be formulated to perform well under such stand-by conditions.
Ordinary ink compositions, which are formulated to be read under natural light, perform well for most uses. However, many inks, including black inks, have reading rates of only about 95% in high speed machine reading applications such as mail sorting and the like. Many known inks also fail to form images having the minimum color intensity required by advanced optical character reading equipment. Fluorescent inks have been suggested to improve the readability of the printed material, as in U.S. Pat. Nos. 4,150,997, 4,540,595, and 4,243,694. These fluorescent compositions, however, are formulated with water or electrolytes for use in continuous printers.
A need therefore exists for ink compositions that can be read by either machine devices or humans, provide higher reading rates, and can be used in an impulse ink jet machine.